1. Field of the Invention
The present invention relates to a radio communication system, and more particularly to a radio communication system intended to reduce transmission power by adopting a diversity method on the transmitting side.
2. Description of the Related Art
It is usually the case in radio communications that radio frequency signals transmitted between a given transmitting station and receiving station are sometimes propagated over different paths; this can produce the phenomenon called "fading", whereby individual component wave interference and the like produces fluctuations in received wave level. Diversity is a technique used to overcome the phenomenon of fading.
One common diversity technique that is widely used is reception diversity, whereby a plurality of antennas is provided on the receiving side for signal reception and the strongest signal is selected, or by addition, in some selected ratio, of signals received by a plurality of antennas in order to produce a strong signal.
Transmission diversity, whereby a plurality of antennas is provided on the transmitting side, is also used in some cases. In such cases, it is necessary to decide, for the plurality of antennas, the ratio at which transmission will take place from each antenna, with reference to the conditions of wave propagation between the transmitting station and the receiving station which are to communicate.
In TDD (time division duplex) communication, whereby transmission and reception are conducted by time-division multiplex communication at the same given frequency, one method for achieving transmission diversity involves receiving radio signals with a plurality of antennas, determining the antennas with high reception levels, and using these antennas for transmission.
This method can be used only when the communication partner has been determined. That is, when bidirectional communication is conducted by TDD communication between designated radio stations, it is possible to select the antenna for transmission on the basis of received signal level data.
In the case of initial transmission, where there is yet no information regarding the location, direction, etc., of the partner station, or when communication is to take place with all of a plurality of stations within the base station area, such as in mobile communication systems, and in similar cases, it is impossible to determine which of the plurality of antennas should be used for transmission, making transmission diversity impossible.
In such cases in the past, the only way to conduct communication with a quality equivalent to that when communication by TDD has been established and transmission diversity by TDD is employed was to increase transmission output.
However, increasing transmission output has cost disadvantages and the additional drawback that interference with other communication systems increases.